2000A&A...360...10R

From regular monitoring of the Double Quasar QSO 0957+561 A,B there is now general agreement on a time delay of about 416 days. This has made it possible to determine the microlensing residual in the light-curve, see Pelt et al. (1998A&A...336..829P). We have used two significant microlensing features: 1) A ``quiet'' period with a variability less than 0.05mag lasting about 8 years, and 2) A change in the residual of 0.25mag during a time interval of about 5 years. The first feature gives a lower limit for the lens mass, M, for a given normalized source radius, r, whereas the second feature gives an upper limit. We have considered the amount of mass in a continuum to be a free parameter with possible values between 0% (all mass in lenses) and 90%. At a significance level of 1% the mass can only be constrained within a rather wide range (10^–6M_☉ to 5M_☉). For the radius R of the source an upper limit of 10¹⁶cm is found, whereas the normalized source radius r is restricted to be smaller than 30. At a level of 10%, however, the range of possible masses is much narrower (2.10^–3M_☉ to 0.5M_☉), and the upper limit of R is about 6.10¹⁵cm, whereas the value of r is restricted to be less than 2. We have used an effective transverse velocity V equal to 600km/s.